1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and a computer program product.
2. Description of the Related Art
Heretofore, because of an increasing demand for high-speed color copiers, tandem-type color copiers have been mostly used in which electrostatic latent image forming units, including photosensitive elements for the four colors, cyan (C), magenta (M), yellow (Y), and black (K), are arranged parallel to one another. In the tandem-type structure, technology for aligning the respective colors is an important issue.
Therefore, most tandem-type color copiers have a function that reduces color deviation by forming predetermined toner patterns on a transfer belt using toner of the respective colors, detecting the toner patterns using optical sensors so as to calculate the amounts of color deviation among the respective colors arisen from different factors such as mis-registration in the main scanning direction and the sub-scanning direction, magnification deviation, or skew, and thus performing feedback correction so that the colors match one another.
Furthermore, this correction process is performed when the power is turned on, when an environment such as temperature changes, or when more than a predetermined number of sheets are printed so that the amount of color deviation is controlled so as to fall within a predetermined range. Among the color deviation amounts, the miss-registration of main/sub scanning direction can be corrected by adjusting the write timing of the laser light projected onto a photosensitive element. The main-scanning magnification can be electrically corrected by adjusting the pixel clock.
A method of mechanical correction and a method for correcting an output image by image processing can be used to improve the skew of laser light that scans with exposure. In the method of mechanical correction, an adjustment mechanism for displacing a mirror inside a laser-light write unit is provided so as to perform correction; however, there are problems in that an actuator such as a mirror displacement motor is necessary for automatic operation, which results in an increase in cost, and the size of the laser-light write unit cannot be reduced.
On the other hand, in the method of correction by image processing, part of an image data is stored in a line memory and the image is read from the line memory while the read position is switched so that an output image is output by being shifted in the direction opposite to the skew, whereby the skew among the respective colors is corrected. In this technique, it is only necessary to add a line memory to an image processing unit in accordance with the correction area; therefore, there is an advantage in that the method can be performed at a relatively low cost compared to the mechanical correction. Furthermore, the method of correction by image processing is also useful as a method for reducing not only skew but also curvature due to the characteristics of a lens inside the laser-light write unit, or the like.
Japanese Patent No. 3715349 (Japanese Patent Application Laid-open No. H09-039294) discloses an image recording apparatus for reducing skew by such image processing. In the image recording apparatus disclosed in Japanese Patent No. 3715349 (Japanese Patent Application Laid-open No. H09-039294), it is detected whether a pixel of interest is in a shift position and, if the pixel pattern around the change point of an adjacent pixel in the main scanning direction at the shift position and the pixel of interest matches a predetermined pattern, the concentration correction corresponding to a combination of the matched pixel pattern and the change position of the adjacent pixel in the main scanning direction is performed on the pixel of interest so that linear noise can be reduced.
However, in the conventional technology, part of an image is stored in a plurality of line memories and the image is read while the read position is switched so that the image divided in the main scanning direction is shifted in the sub-scanning direction to reduce skew among the respective colors; therefore, a line image, which should be originally smooth in the main scanning direction, is shifted by one dot before and after the connection position (division position) at which the image is shifted in the sub-scanning direction, which causes an negative effect such that jaggy is noticeable. That is, in conventional technology, although misalignment itself is reduced, there is a problem in that the smoothness of line images cannot be maintained.
Heretofore, a technology for a smoothing process has been known as a technology for correcting the jaggy of an image. In the smoothing process, the jaggy at the edge portion of a character or a line image is detected by referring to pixels in the vertical and horizontal directions, and a pixel having a size smaller than that of a pixel of an input image is interpolated so as to convert the image into a smooth image. In the smoothing process, a process of matching the input image and a specific pattern is performed on the image and, if the input image matches the pattern, correction is performed to replace the concentration of pixels, whereby the resolution is improved in terms of appearance, and the edge of a character or a line image is made smooth. However, gradation is degraded if the smoothing process is performed on an image that has undergone a halftone process such as a dither image, in which shading is represented in accordance with the degree of density, for example, a photographic image, or the like; therefore, the process is performed on only line image portions, and image areas that have undergone a halftone process are not subjected to the process.
In a monochrome image, the concentration replacement correction by the smoothing process is performed on a portion of a character or a line image that has undergone a binarization process, and such a correction process is not performed on an image such as a dither image that has undergone a halftone process; therefore, it is advantageous that the gradation of the image that has undergone the halftone process is not degraded. However, a character or line image portion of a color image is output in more than two colors in a superimposed manner, for example, a red line is output in yellow and magenta in a superimposed manner, and the halftone process is performed on each color in order to obtain gradation; therefore, they are not subjected to a conventional smoothing process and a jagged portion of a line image cannot be made smooth. That is, if a skew correction is performed on an image that is obtained by performing a halftone process on an input image, it is not possible to make a jagged portion of a line image smooth.
If the smoothing process is performed after the skew correction is performed on an image that is obtained by performing a halftone process on an input image, because the process of matching the input image and the specific pattern is performed during the smoothing process, the correction area of the smoothing process is determined in accordance with the size of the pattern matching. Therefore, the concentration replacement correction is performed only within a limited area. As a result, the smoothing process cannot be sufficiently performed on the jagged portion caused by the skew correction.